Amplifier



Sept. 19, 1933. v J E, B 1,927,082

AMPLIFIER Filed June; 30 1932 FREQUENCY lNVE/VTOP J 5 CORE/N ATTORNEYPatented Sept. 19, 1933 v AMPLIFIER Joseph E. Corbin, Jackson Heights.51. Y., as-

signor to Bell Telephone Laboratories Incor-' yporated, New York, N. Y.,a corporation of New York ApplicationJune so. 1932. Serial No. 620,087,.

2 Claims; (01. 179-171) This invention relates to amplifiers using spaceoften obtained from an impedance inserted in the anode circuit of thedevice. McNally Patent 1,848,187, March 8, 1932, shows how independentresistors in the anode'circuit improve the operation and balance of suchcircuits when applied to devices arranged in push-pull or balancedrelation. Heretofore, the bias resistors have been shunted by condenserswhereby a filter is formed which tends to eliminate variations in gridbias due to signal or noise variations.

The present invention has as an object the improvement of the operationof amplifiers involving circuits of the type noted.

More specifically it is an object of this invention to improve theresponse of balanced amplifiers at low frequencies.

Another object is to reduce even order products of modulation orparallel singing in amplifiers of the balanced type.

A feature of the invention is a single capacity connected between thecathodes of the space discharge devices of a balanced amplifier in shuntto the bias resistors.

The invention together with additional objects and features will bebetter understood from the following description and attached drawingforming a part thereof in which Fig. 1 is a diagrammatic sketch of oneform the invention may take; and

Fig. 2 is a graph showing the response of the amplifier with difierentvariations of capacity connected between the cathodes of the devices.

Referring now to Fig. 1 a transformer 2 having a primary winding 1 and adivided secondary winding 3 is shown with the outside terminals of thesecondary winding connected to the grids 4 and 5 of space dischargedevices 6 and '7.

The primary winding 1 of transformer 2 may be connected to any source,the output of which is to be amplified such as a preceding amplifierstage, a microphone, a telephone line, a radio receiving system, etc.

Devices 6 and "1 are illustrated as being of the well-known heater typehaving filaments 8 and 9 connected to a suitable source of current 10,cathodes 11 and 12 and anodes 13 and 14. It is to be understood,however, that the invention is not limited to tubes of the heater typebut may be applied regardless of the type of cathode.

Anode current" is obtained from source 15 shown as a battery but it maybe any direct current source such as rectified alternating current.

Individual bias resistors are shown at 16 an i7. shunted by condenser18. v I

Anoutput transformer 21 has a divided primary winding connectedin thewell-known manner to the anodes 13 and 14 and its mid-point connected tosource 15-. The secondary winding of transformer 21'may be connectedtoany utilization circuit.

The input circuits of the amplifier may be traced from the mid-point ofsecondary winding 3 through either half of the winding to grids 4 or 5,cathodes 11 or 12, resistors 16 or 17 to the mid-point of winding 3.

g The anode circuits may be traced from grounded source 15 to mid-pointof primary winding of transformer 21, anodes 13 and 14, cathodes 11 and12 to ground at 20. The IR drop through the resistors 16 and 17 providesthe bias for grids 4 and 5 in the well-known manner.

It will be noted that the resistors are common to the grid and anodecircuits of each of the devices 6 and 7. They are also common to theinput and output circuits of the amplifier considered as a unit. Now,since the signal voltage in the anode circuit is in phase opposi ion tothe signal voltage in the grid circuit, if condenser 18 were not presentthe signal currents in passing through the resistors would causevoltages to be applied to the grids opposing the signal voltages andhence cause a reduction in the amplifier gain. Condenser 18, however,effectively shortcircuits the bias resistors at signal frequencies andhence this negative regeneration does not occur except at suchfrequencies that condenser 18 is not an adequate short circuit.

7 The effect of different capacity values for condenser 18 is shownin'Fig. 2. Maximum gain at all frequencies is obtained by making thecapacity theoretically infinite. That is, making it so large that itsimpedance to low frequencies is very low compared to that of theresistors. This is shown in curve E where it is seen that the amplifiergain is substantially uniform throughout the frequency range of theamplifier. Curves D and B illustrate amplifier response with lowercapacity values for the shunting condenser and curve A the response withcapacity zero or the condenser eliminated. Here the negativeregeneration reduces the amplifier gain throughout the entire range.

It is desired to point out that an inductance may be used with capacity18 either in series or in parallel to provide a circuit resonant orantiresonant at certain frequencies whereby the gain of the amplifier atsuch frequencies may be increased or decreased as desired.

The second function of condenser 18, that is, the reduction of evenorder products is closely allied with the function just described. Aspreviously pointed out the individual resistors are common to both thegrid and anode circuits and also to the input and output circuits of theamplifier as a whole. It is also well known that currents arising fromeven order modulation or parallel singing exist in the common branchesof both circuits.

If condenser 18 were made up of two parts, each part shunting only itsindividual resistor, the even order products currents would be shuntedthrough these condensers and hence not pass through the resistors.However, with condenser 18 connected from cathode to cathode the evenorder products must pass through resistors 16 and 17 and hence the dropthrough the resistors due to these currents is impressed on the gridcircuits in such phase as to oppose their production. Negativeregeneration of these even order components, therefore, occurs with asubstantial decrease in amplitude if not complete elimination. Further,the negative regeneration is constant at all frequencies and noadjustment is necessary regardless of the frequency of the disturbingcurrents.

It is to be understood that whereas the invention has been described inconnection with but one embodiment it is to be limited only by the scopeof the appended claims.

What is claimed is:

1. An amplifier comprising space discharge devices arranged in push-pullrelation, input and output circuits for said amplifier, a resistanceelement associated with each said device and common to both the inputand output circuits thereof, whereby currents of even order modulationproducts set up a voltage in the input circuit which tends to neutralizesuch currents and a single condenser shunting both said resistanceelements.

2 An amplifier comprising space discharge devices arranged in push-pullrelation, input and output circuits for said devices, a branch common toboth the input and output of each said devices, a resistance connectingthe cathode of each said device to said common branch, a condenser inshunt to both said resistances forming a bypass around said resistancesfor desired waves,

said resistances forming an impedance to currents of even ordermodulation products to thereby set up a voltage ten-ding to neutralizesaid even. order products.

JOSEPH E. CORBIN.

